Feature performance involving special source of expertise (6)
I believe the answer is:
aspect
'feature' is the definition.
(aspect is a kind of feature)
'performance involving special source of expertise' is the wordplay.
'performance' becomes 'act' (act is a kind of performance).
'involving' is an insertion indicator.
'special' becomes 'sp'.
'source of expertise' becomes 'e' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'sp'+'e'='spe'
'act' going around 'spe' is 'ASPECT'.
Can you help me to learn more?
(Other definitions for aspect that I've seen before include "View or facet" , "Characteristic to be considered" , "Particular or distinct element, of a problem say" , "particular view" , "Distinct part or element" .)